Re: [edk2-devel] [PATCH v5 1/6] MdeModulePkg/PciHostBridge: io range is not mandatory

["Ni, Ray" <ray.ni@intel.com>]

> 
>   Hi,
> 
> > > I'd say the risk for regressions is rather low, though, given that it
> > > only affects configurations that would fail PCI resource allocation
> > > today. Or am I missing something?
> > >
> > > In any case, the PCIe spec is clear about this: I/O space is optional,
> > > and we need to incorporate this into the generic code at *some* point.
> > > It makes no sense for every individual platform to keep adding these
> > > hacks.
> > Do you know how Linux handles this?
> > Can Linux allocate resource for PCI(E) devices? How does it deal with the IO type?
> 
> Yes.  Details depend a bit on the specific configuration, but in general
> linux will try assign io address space to pcie root ports and devices
> plugged into those ports.  A failure is not considered fatal though.

An error message and continue? 

> 
> A more common case than the pci root bridge not supporting io address
> space at all is having more than 16 pcie root ports.  Given io bride
> windows are 1k in size and we have 16k total there is simply not enough
> io address space in that case, so some of the root ports stay without
> io and linux is fine with that.

Does it have some certain policy that IO resource for first root bridge should
be satisfied?

> 
> > Why changing the MdeModulePkg's IncompatiblePciDevice driver can avoid
> > stalling the patch series?
> > I feel it's enough to just change the OvmfPkg version.
> 
> It's not much of a problem for ovmf even without such an update,
> typically the devices used with microvm don't have io bars in the first
> place.
> 
> Also note that without this series pcie devices are not supported at all
> on microvm, so not supporting all devices initially wouldn't be a
> regression.
> 
> I'll look into it in any case.

The safest way is to change OVMF now.
Add @Nong, Foster and @Albecki, Mateusz for comments.

> 
> take care,
>   Gerd
> 
> 
> 
> 
>